This invention relates to apparatus for detecting the thickness of sheets as they are moved past the apparatus, and is especially applicable to the detection of banknotes where the thickness of two or more superimposed banknotes should be distinguished from that of a single banknote.
Machines for detecting the simultaneous passage of two banknotes where banknotes should normally pass singly are known, and one example of an optical detector is in commonly assigned British Pat. No. 1,344,986, the detector responding to the opacity of the note or notes; this is inadequate if the notes are greasy or soiled. Mechanical thickness detectors are also known using a roller whose axis is displaced by the passage of a banknote or banknotes to cause an electrical contact to be made or broken if the displacement is such as to indicate the passage of superimposed banknotes. Apparatus for gauging laminar materials, using nip rollers and a variable-reluctance transducer, is disclosed in commonly assigned British Pat. No. 1,497,181.
However, none of the machines has yet been able to compensate for long-term variations in the detecting heads and in the control circuitry which will tend to change the threshold value of the banknote thickness, i.e. the thickness at which the detector will signal the passage of a double banknote. In the case of the mechanical detector using a roller, this long-term drift could be caused by mechanical movement between the pivot of an arm attached to the axis of the roller, and the surface on which the note is placed as it is moved under the roller. This potentially degrades note thickness measurement if the output of the detector is compared against a fixed threshold level in the output stages of the control circuit. Mechanical stability of the order required for measuring banknote thicknesses, for example 0.005 cm to 0.015 cm, is difficult to achieve under reasonable manufacturing and wear rate conditions.